This invention relates to single sideband (SSB) frequency division multiplex (FDM) equipment used for telecommunication systems and more particularly to a redundant carrier frequency selection arrangement for such FDM communication systems.
The SSB/FDM equipment is more commonly referred to as an FDM system and normally handles large numbers of voice signals (individual telephone channels), which are combined into a multichannel broadband signal for transmission over a given distance. At the receiving end the FDM equipment separates the multichannel signals back into individual voice signals.
Those portions of FDM equipment that handle more than 12 channels are constantly provided with redundant or backup equipment which automtically takes over should the normal equipment fail. One such sub-system in FDM systems is the carrier supply. The carrier supply functions to provide numerous outputs of different frequencies for use as carriers and pilot signals. These signals of different frequencies are usually all derived from a common master oscillator which may or may not be synchronized to an external frequency standard. Two such complete carrier sources are normally provided within the carrier supply system selector circuits.
In practical systems a carrier supply must be provided initially with an FDM communications system even though it is equipped with a small telephone traffic capacity. Such systems have relatively few units requiring redundancy. The units requiring redundancy are known as translators, modulators, demodulators and modems and are referred to herein as FDM communication equipment requiring a carrier or more simply as FDM equipment.
The normal lumped carrier selection arrangement of the prior art wherein the selection of one or the other of redundant carrier frequencies is accomplished at a central carrier supply arrangement has the following disadvantages:
1. When a redundant frequency switch fails, all of the FDM equipment connected to that switch are rendered inoperative; PA1 2. The switch is costly and requires significant power since the switch must handle sufficient power to provide selected redundant carrier to all of the FDM equipments connected to that switch; and PA1 3. The single conductor carrying the selected redundant carrier frequency from the switch to the plurality of FDM equipments is subject to being short circuited or open circuited and thereby causing outage in all the FDM equipment connected with this single redundant carrier frequency conductor.